Regarding filters for use in mobile communication terminals, miniaturization and weight reduction are desired. Furthermore, it is desired that the filters have a small loss at a passband and large attenuation at a frequency band outside of the passband (hereinafter, referred to as a non-passband frequency band) and have a sharp frequency-characteristic change at a boundary between the passband and the non-passband frequency band.
In addition, regarding duplexers that separate signals of a transmission frequency band from signals of a reception frequency band, miniaturization and weight reduction are desired. Transmission filters used in the duplexers are desired to have a low loss at a transmission frequency band and large attenuation at a reception frequency band and to have a sharp frequency-characteristic change at a boundary between the transmission frequency band and the reception frequency band. Reception filters used in the duplexers are desired to have a low loss at the reception frequency band and large attenuation at the transmission frequency band and to have a sharp frequency-characteristic change at a boundary between the reception frequency band and the transmission frequency band. Furthermore, it is desired that the duplexers have a preferable isolation characteristic between a transmission terminal and a reception terminal.
Duplexers including dielectric resonator filters had been used. However, due to a demand for miniaturization, duplexers including surface acoustic wave (SAW) filters and duplexers including film bulk acoustic resonator (FBAR) filters are used.
As disclosed in Japanese Unexamined Patent Application Publication No. 2002-176337, striplines, distributed-constant lines, and chip components such as a chip inductor and a chip capacitor are arranged between a transmission filter and a reception filter as a matching circuit in a known SAW-filter-including duplexer in order to adjust impedance of the transmission filter and the reception filter. According to Japanese Unexamined Patent Application Publication No. 2002-176337, since the matching circuit is arranged between the transmission filter and the reception filter, sufficient miniaturization is not achieved.
In Japanese Unexamined Patent Application Publication No. 2004-336181, a SAW device having excitation electrodes arranged on a piezoelectric substrate is mounted at a cavity portion of a package main body. The electrode patterns arranged on the piezoelectric substrate are connected to terminals of the package using a wire boding technique and the cavity portion is sealed with a cap or the like, whereby a SAW filter is created. The SAW filter is miniaturized by including a matching circuit in the package main body.
In this case, an inductance component of a bonding wire is effectively used by connecting parallel arms forming the SAW device and the terminals of the package using the bonding wire, whereby an attenuation characteristic can be improved at a non-passband frequency band of the SAW filter.
To further miniaturize the package, a reduction in a space and a height needed for wire bonding by employing flipchip mounting of a SAW device, which is formed on a substrate, on a circuit board positively using a chip size package (CSP) technology has been suggested. Since an inductance component constituted by a bonding wire no longer exists in the flipchip mounting, an attenuation characteristic can be improved at a non-passband frequency band by providing a line having an inductance component on the circuit board. According to Japanese Unexamined Patent Application Publication No. 2003-198325, a matching circuit and a line having an inductance component are arranged in a package main body so that interference is not caused between the matching circuit and the line. More specifically, the matching circuit is formed in an inner layer of the package, whereas the line having the inductance component is routed apart from the matching circuit and is connected to a ground through a castellation arranged at a package outer periphery. In addition, a ground layer is arranged over the matching circuit to suppress interference between the matching circuit and other circuits. Accordingly, sufficient height reduction and miniaturization are not achieved.
FIG. 11 is a diagram showing a configuration of a duplexer 1 according to the related art. A known duplexer 1 includes a first filter 2 and a second filter 3, which have different passbands. The first and the second filters 2 and 3 are connected to a common node P. An antenna terminal 4 is also connected to the common node P. For example, suppose that the first filter 2 is a filter (hereinafter, referred to as a “TX filter 2”) that passes signals of a transmission frequency band, whereas the second filter 3 is a filter (hereinafter, referred to as a “RX filter 3”) that passes signals of a reception frequency band. In that case, a transmission signal supplied to a transmission signal terminal 5 from a transmission circuit, not shown, is supplied to an antenna, not shown, through the antenna terminal 4 after propagating through the TX filter 2 and is transmitted to another communication device. In addition, a reception signal received by the antenna and input to the antenna terminal 4 is supplied to a reception circuit, not shown, from a reception signal terminal 6 after propagating through the RX filter 3.
In the duplexer 1, for example, part of the transmission signal supplied from the transmission circuit to the transmission signal terminal 5 passes through the TX filter 2 and leaks to the RX filter 3 from the common node P. Accordingly, a matching circuit 7 is provided between the antenna terminal 4 and the filters 2 and 3, more particularly, between the antenna terminal 4 and the common node P. The matching circuit 7 can adjust impedance so that impedance of the transmission circuit from the antenna terminal 4 reaches an infinite value at the reception frequency band and that impedance of the reception circuit from the transmission circuit reaches an infinite value at the transmission frequency band.
Techniques according to the related art employ a method for providing a wire and a line having an inductance component between parallel arms forming a SAW device and a ground to miniaturize a SAW filter and keep a preferable attenuation characteristic at a non-passband frequency band. Duplexers also employ the above-described techniques to improve an attenuation characteristic and an isolation characteristic at a non-passband frequency band. However, unlike duplexers manufactured using the wire bonding technique, duplexers manufactured using the CSP technology cannot use a bonding wire having an inductance component. Thus, a line having an inductance component has to be provided on a circuit board.
Nevertheless, since input/output electrodes, a ground electrode, and a matching circuit are arranged on the circuit board constituting a duplexer, it is undesirably difficult to provide a sufficiently long line having an inductance component, due to which desired attenuation and isolation characteristics cannot be satisfied.